Silicon solar cells with aluminum-magnesium alloy low resistance contacts

ABSTRACT

An Al-Mg alloy is mixed with a Ni-Sb alloy or Al, in powder form, to form a thick-film metallizing paste useful for making low resistance electrically conductive contacts to a silicon solar cell coated with Si 3  N 4 .

This is a division of application Ser. No. 210,553 filed Nov. 26, 1980,now U.S. Pat. No. 4,347,262.

FIELD OF THE INVENTION

The present invention relates to the art of converting solar energy intoelectrical energy by means of a silicon solar cell, and moreparticularly to a thick-film paste composition for making low resistancecontacts to such a cell.

BACKGROUND OF THE INVENTION

It is well known that radiation of an appropriate wavelength falling ona P-N junction of a semiconductor body serves as a source of externalenergy to generate hole-electron pairs in that body. Because of thepotential difference which exists at a P-N junction, holes and electronsmove across the junction in opposite directions and thereby give rise toflow of an electric current that is capable of delivering power to anexternal circuit. Most solar cells are in the form of a silicon waferwhich has been metallized, i.e., provided with metal contacts which areelectrically conductive.

To provide a low cost method of generating an electrical current fromthe P-N junction region of the silicon wafer, it is common practice tometallize the wafer by a screen printing and firing sequence.Commercially available metallizing inks which are employed fordepositing contacts on the surface of the wafer generally contain ametal powder, a finely divided glass frit, and an organic vehicle.Typical metal powders are those of silver, aluminum, nickel, gold, orcopper, or alloys of these with precious metals, including platinum andpalladium.

There is extensive use of Si₃ N₄ in solar cell technology as ananti-reflection coating, which also serves as a masking protectivelayer. It has good adhesion and stability when deposited on silicon. Ina specific embodiment, silicon solar cells are coated with Si₃ N₄ on thefront N-type side as an anti-reflection coating, and, in the process,the back P-type side also becomes coated with Si₃ N₄. In order to makeelectrical contact to the underlying silicon substrate, an etchant stepmust be employed. State of the art technology requires that the Si₃ N₄be removed where contact is made, the front side etched in pattern formfor application of the front side contact, and the back side similarlyetched for application generally of a large area backside contact. Therewould be a cost savings if this etching step could be eliminated. It hasnow been found that aluminum-magnesium alloys can perform this functionwhen contained in metallizing paste formulations.

SUMMARY OF THE INVENTION

In terminating Si₃ N₄ -coated solar cells with base metal contacts suchas of Ni-Sb alloys or aluminum, improvement in electricalcharacteristics are obtained, and the firing window, i.e., thetemperature range for satisfactory firing, is widened, when 50 Al:50 Mgalloy powder is incorporated in the metallizing paste. In a specificembodiment, the invention resides in a thick-film metallizing paste foruse in providing low resistance electrically conductive contacts(terminations) to a silicon solar cell coated with Si₃ N₄, having aP-type and an N-type region, and a P/N junction, said paste comprisingan organic vehicle containing, in particle form, a mixture of a majoramount of a metal powder, e.g., aluminum or a Ni-Sb alloy, a minoramount of finely divided glass frits, and a small amount of a 50 Al:50Mg alloy. The invention further resides in the process of metallizingthe cell, and in the resulting product. The metallization processtypically comprises screen-printing one surface of the cell with themetallizing paste of this invention and firing at a temperature of atleast 500° C.

DETAILED DESCRIPTION

This application is related to copending application Ser. No. 218,493,"Solar Cell Metallizations Comprising a Nickel-Antimony Alloys" filedDec. 19, 1980, now U.S. Pat. No. 4,342,795. The present invention isdemonstrated by the Examples which follow.

EXAMPLE 1

A front surface textured silicon solar cell, constructed by applyingN-type impurities 0.4 to 0.5 microns in depth into a P-type siliconwafer that had been etched to form pyramidal texture on the diffusionside, and having a Si₃ N₄ anti-reflection coating, was metallized toprovide metal contacts or terminations. The metallizing paste wasscreen-printed on the N surface of the wafer and was composed of anorganic vehicle (ethyl cellulose/dibutyl phthalate in terpineol), NiSballoy, glass frits, and a minor amount of a 50 Al:50 Mg alloy.Composition of the glass frits in % by wt, was PbO 83, PbF₂ 4.9, B₂ O₃11, and SiO₂ 1.1. Three samples of paste were prepared, and theterminations were nitrogen-fired. The proportions of the metalcomponents were varied, as indicated in the following tabulations:

    ______________________________________                                        Paste #   NiSb        Glass   50 Al:50Mg                                      ______________________________________                                        1         88          12      0                                               2         83          12      4.7                                             3         79          12      9.3                                             ______________________________________                                    

The soldered electrical characteristics of the nitrogen-fired solarcells are listed in Table I.

                  TABLE I                                                         ______________________________________                                        Termi-                                                                              Firing     V.sub.oc                                                                              Fill   R.sub.Series                                                                         R.sub.shunt                            nation                                                                              Temp °C.                                                                          (mv)    Factor (ohm)  (ohm)                                  ______________________________________                                        1     500        --      --     Large  (Infinity)                             1     550        580     0.57   1.7    "                                      1     575        520     0.31   1.4    "                                      2     500        560     --     8.6    "                                      2     550        595     0.50   1.2    "                                      2     575        595     0.54    0.73  "                                      3     500        590     --     6.0    "                                      3     550        600     0.50   2.0    "                                      ______________________________________                                    

Table I demonstrates that the termination not containing the 50 Al:50 Mgalloy has a window with lower firing temperature near 550° C., whilethis window is extended downwardly to at least 500° C. when the alloy ispresent. Table I also demonstrates that the electrical characteristicsof the solar cell, i.e., series conductivity and Voc (voltage generatedacross the cell when illuminated by one sun with no current flowingthrough cell) are enhanced when use is made of this alloy. Thisdemonstrates that the 50 Al:50 Mg alloy was effective in penetrating theSi₃ N₄ coating and making contact with the N-type region.

EXAMPLE II

The 50 Al:50 Mg alloy was incorporated into 2 of 3 samples of athick-film aluminum base metallizing paste, and these were applied byscreen-printing to the back P-type surface of a silicon solar cellcoated with Si₃ N₄. The resulting terminations were air-fired. Table IIshows the back contact resistance in ohms of the solar cells soterminated (making use of a two-probe measurement between parallelconductor lines on the P surface of the silicon cell).

                  TABLE II                                                        ______________________________________                                                                  Back Contact Resistance Ohms                        Paste       50 Al:        (Air Firing Temperature)                            #    Al     50 Mg   Glass 500° C.                                                                      550° C.                                                                      610° C.                                                                      650° C.                    ______________________________________                                        1    81.8   5.8     12.5  --    --    10    10                                2    87.5   0       12.5  --    --    50M-  500K.sup.2 -                                                            100M.sup.1                                                                          1M                                3    66.3   8.8     25.0  --    --    52    50                                ______________________________________                                         .sup.1 M = 1 million                                                          .sup.2 K = 1,000                                                         

It is evident from Table II that there is a significant reduction incontact resistance of formulations to which 50 Al:50 Mg had been added,compared to that where the 50 Al:50 Mg was absent.

The foregoing Examples are illustrative only. It is to be understoodthat other vehicles, other metal powders, other glass frit compositionsand other Al:Mg alloys may be employed, to the extent that they functionto form a thick-film metallization paste for use in providing lowresistance electrically conductive contacts to a silicon solar cellcoated with Si₃ N₄. While screen printing is disclosed above, othermethods of application to the substrate such a brushing, spraying,stamping, etc. could be used. The organic vehicle employed in theprinting paste is generally employed in an amount such that the printingpaste will contain 70-90% solids and 10-30% vehicle. A number of inertliquid vehicles commonly used in the art are described in greater detailin U.S. Pat. No. 4,172,919, column 4, lines 3-28, which lines areincorporated by reference herein.

I claim:
 1. A silicon solar cell coated with Si₃ N₄ having a P-type andan N-type region and a P-N junction, the N surface of which has beenmetallized by (1) screen-printing thereon a thick film paste comprisinga mixture of finely divided particles of a major amount of a metalpowder, a minor amount of glass frits and a small amount of a 50 Al:50Mg alloy, dispersed in an organic vehicle, and (2) firing the printedsurface at a temperature of at least 500° C. to form electricallyconductive contacts thereon.
 2. A silicon solar cell coated with Si₃ N₄having a P-type and an N-type region and a P-N junction, the P surfaceof which has been metallized by (1) screen-printing thereon a thick filmpaste comprising a mixture of finely divided particles of a major amountof a metal powder, a minor amount of glass frits and a small amount of a50 Al:50 Mg alloy, dispersed in an organic vehicle, and (2) firing theprinted surface at a temperature of at least 600° C. to formelectrically conductive contacts thereon.
 3. The solar cell of eitherclaim 1 or 2 in which the metal powder is aluminum.
 4. The solar cell ofeither claim 1 or 2 in which the metal powder is NiSb alloy.